OK. So if it's a light leak through the pressure plate of the insert, how does the light get into the fixed back in the first place?(posted 8667 days ago)
Nigel, I suggest you check the fabric light seal that covers the top edge of the mirror. It looks to me as if light might just be sneaking down the back of the mirror, through the viewfinder.
Do you only use the WLF? A prism finder might cure the problem.